1. Field of the Invention
This invention pertains generally to optical imaging devices and methods and more particularly to an apparatus and method for high-speed, high-contrast, real-time, one-, two- and three-dimensional imaging enabled by differential interference contrast time encoded amplified microscopy that is particularly suited for image acquisition of transparent media without staining.
2. Description of Related Art
Optical imaging modalities are widely used for detection, inspection, and diagnostics in numerous industrial, biomedical, and scientific applications. For example, optical microscopy is relied on extensively in hematology for identification of diseased blood cells. While preliminary screening can be performed by techniques such as impedance measurement and laser scattering in a flow cytometer, optical imaging with a microscope that is performed manually by a technician is used for accurate identification of the suspected cells. Similarly, metallurgy and semiconductor processing also rely heavily on optical imaging modalities for imaging of surface scratches, lines and edges, defects, and contaminations inside the material under test.
Another fundamental difficulty in the imaging of biological cells with present optical imaging modalities arises from the fact that cells are transparent and have nearly no contrast with respect to their aqueous surrounding (index contrast typically=0.05). Because of this, cells are usually stained with dyes to provide contrast with the surrounding medium and also between structural components within a single cell.
Some imaging modalities, such as differential interference contrast microscopy (DIC) and phase-contrast (PC) microscopy, can capture images of transparent objects without the need for chemical staining. Although a DIC microscope gives high resolution images with high clarity, they have slow frame rates, limited by the CCD or the CMOS image sensor array used in all conventional microscopes. Therefore, DIC microscopy cannot be used for applications that require monitoring of dynamic samples in real-time with a high throughput such as flow cytometry.
High-throughput imaging in such applications is highly desirable, but extremely challenging. For instance, high-throughput screening of biological cells that show nearly no contrast with respect to their aqueous surroundings makes finding of rare diseased cells in a large population of healthy cells very difficult. However, conventional techniques that are used for performing this task rely on CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) image sensors. As a result, the image acquisition throughput is limited by that of CCD and CMOS cameras. More importantly, the shutter speed of even the fastest cameras is too slow, resulting in significant blurring of images during high-speed screening.
The recently introduced imaging technology known as serial time-encoded amplified imaging (STEAM) overcomes limitations in conventional imaging and provides ˜1000 times higher frame rates and shutter speeds than available with current image sensors (i.e., CCD and CMOS cameras). This system replaces the conventional CCD/CMOS camera with a single-pixel photodetector. As a result, the trade-off between sensitivity and frame rate is overcome.
The STEAM approach exploits an amplified space-to-time mapping technique to encode the spatial information of an object into a one-dimensional (1D) serial time-domain optical waveform and optically amplifies the image, simultaneously. The high-speed capability of a STEAM imager enables its use for applications in which high-throughput screening of an object is of interest, such as blood cell screening. However, it cannot be used on transparent samples such as cells but rather it is limited to opaque samples or samples with high refractive index or absorption contrast. Therefore, the STEAM imager is inadequate for imaging of transparent objects such as biological cells due to their poor refractive-index contrast with their surroundings. While these objects can be stained with dyes to increase their contrast, chemical staining often requires careful sample preparation and is unsatisfactory.
Accordingly, there is a need for an imaging apparatus and method that can provide contrast without the need for staining and also one that has a very high frame rate and shutter speed to allow imaging of large numbers of cells during flow in a reasonably short period of time. The present invention satisfies these needs as well as others and is generally an improvement over the art.